The use of unmanned aerial vehicles such as airplanes or helicopters having one or more propellers in a variety of applications is increasingly common. Such vehicles may include fixed-wing aircraft, or rotary wing aircraft such as quad-copters (e.g., a helicopter having four rotatable propellers), octo-copters (e.g., a helicopter having eight rotatable propellers) or other vertical take-off and landing (or VTOL) aircraft having one or more propellers. In most unmanned aerial vehicles, each of the propellers is powered by one or more rotating motors or other prime movers. The motors and propellers may be provided in propulsion units or modules that are physically joined to a frame or other structure of an unmanned aerial vehicle, e.g., a fuselage, a wing, or another portion of the vehicle, and electrically and/or mechanically coupled to one or more other systems or components, including but not limited to computer-implemented control systems or modules. The net effects of the operation of such propulsion units cause an unmanned aerial vehicle to travel in one or more directions and/or be held aloft thereby.
Propulsion units that are outfitted to unmanned aerial vehicles typically include motors having shafts with fixed axes of orientation and propellers of fixed shapes, with the motors being configured to rotate the propellers about the fixed axes of orientation by the shafts. Typically, a level of force (e.g., lift and/or thrust) provided by a propulsion unit having a motor and a propeller may be modified either by varying a speed of the motor within a safe operating range (e.g., from a full stop condition to a maximum rotational or angular velocity), or by varying pitch angles (or angles of attack) of blades of the propeller. Propulsion units that are outfitted to unmanned aerial vehicles are typically not configured, however, to change either their respective gimbal angles, e.g., angles of their axes of orientation about which the propellers rotate, and along which the propulsion units are configured to generate force, or the shapes of their respective propellers, during operation.
Sound is kinetic energy released by vibrations of molecules in a medium, such as air. In industrial applications, sound may be generated in any number of ways or in response to any number of events. For example, sound may be generated in response to vibrations resulting from impacts or frictional contact between two or more bodies. Sound may also be generated in response to vibrations resulting from the rotation of one or more bodies such as shafts, e.g., by motors or other prime movers. Sound may be further generated in response to vibrations caused by fluid flow over one or more bodies. In essence, any movement of molecules, or contact between molecules, that causes a vibration may result in the emission of sound at a pressure level or intensity, and at one or more frequencies. Properties of sound emitted by unmanned aerial vehicles during operation (e.g., sound pressure levels or frequency spectrums of such sounds) are determined based on operating characteristics of the aerial vehicles, such as motor speeds or attributes of the propellers rotated thereby.